Suppression of transients in communications across an isolation barrier

ABSTRACT

Common mode transient immunity for an isolation system is improved by using a common transient suppression circuit coupled to a receive circuit to suppress transients in signals received by the receive circuit that were transmitted from a transmit side of the isolation barrier using optical, magnetic, inductive, or other mechanisms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 61/918,416 filed Dec. 19, 2013, whichapplication is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

This application relates to isolation barriers and more particularly tosuppressing common mode transients in communications across isolationbarriers.

2. Description of the Related Art

Isolation barriers can be found in many applications such as voltageconverters. Isolation barrier refers to an electrical isolation betweentwo domains. Such isolation may be needed because during normaloperation a large DC or transient voltage difference exists between thedomains. For example, one domain may be “grounded” at a voltage which isswitching with respect to earth ground by hundreds or thousands ofvolts. Another reason for such isolation is based on safety, even whenthe expected voltage difference between the domains is small in normaloperation. An example of this would be in biomedical applications, whereelectrodes are taped to a patient's body, safety concerns demand anextra layer of protection between the patient and the ground of themeasurement device, despite the fact that the measurement device issupposed to be properly grounded.

Isolation barriers typically consist of layers of dielectrics with goodbreakdown properties. Communication across isolation barriers iscommonly done using optical (opto-isolators), inductive (transformer)solutions, or using capacitive isolation circuitry to transmitinformation across isolation barriers. However, such communication issusceptible to common mode transients that can interfere with theaccuracy of the information transmitted across the channel.

Thus, it would be desirable to provide isolation technology with greaterimmunity to common mode transients.

SUMMARY OF EMBODIMENTS OF THE INVENTION

Accordingly, in one embodiment an apparatus includes a receive circuitto receive signals from an isolation communication channel used tocommunicate across an isolation bather and to provide a receive circuitoutput signal. A common mode transient suppression circuit is coupled tothe receive circuit to suppress transients in the receive circuit outputsignal.

In another embodiment, a method includes receiving a signal transmittedto communicate across an isolation barrier; and suppressing transientsin the received signal having an event length less than a predeterminedlength of time.

In another embodiment, an isolation apparatus includes a transmitcircuit coupled on a transmit side of an isolation barrier to receive aninput signal and to supply a signal to be communicated to a receivecircuit on a receive side of the isolation barrier. The isolationapparatus further includes the receive circuit to receive the signal ona receive side of the isolation barrier and a common mode transientsuppression circuit coupled to the receive circuit to suppresstransients in the signal received by the receive circuit if thetransients have an event length less than a predetermined amount oftime.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 illustrates a gate driver circuit with an isolation barrier.

FIG. 2 illustrates a timing diagram associated with the gate drivercircuit of FIG. 1.

FIG. 3 illustrates a timing diagram of transients that may be associatedwith the gate driver circuit of FIG. 1.

FIG. 4 illustrates an opto-isolator and sources of transients that maybe associated therewith.

FIG. 5 illustrates timing that may be associated with transient events.

FIG. 6 illustrates an exemplary embodiment of a gate driver circuit witha transient suppression circuit.

FIG. 7 illustrates an exemplary embodiment of a transient suppressioncircuit.

FIG. 8 illustrates operation of the transient suppression circuit ofFIG. 7 in the presence of transients.

FIG. 9 illustrates an opto-isolator with a transient suppression circuitaccording to an embodiment.

FIG. 10 illustrates a transformer based isolation system that may beused in place of the opto-isolator with a transient suppression circuitaccording to an embodiment.

FIG. 11 illustrates a capacitive based isolation system that may be usedin place of the opto-isolator with a transient suppression circuitaccording to an embodiment.

FIG. 12 illustrates a Programmable Logic Controller with transientsuppression circuits according to an embodiment.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

In isolated gate driver circuits for switching power supplies there is aneed for circuits that have high common mode transient immunity (CMTI).Logic high/low CMTI is the maximum tolerable change in common modevoltage over time to ensure that the output remains in a valid high/lowstate. In the past, CMTI levels of 25-50 kV/μs have been adequate, butwith emerging new power FET technologies, levels up to 200 kV/μs may berequired. In a switching power supply common mode transients may show upas a transient in the gate driver when the high common mode eventoccurs. That problem is illustrated with reference to FIGS. 1 to 3.Referring to FIG. 1, a switching power supply system 100 is illustrated.In the system 100, the power MOM-ET transistors M1 and M2 are on atdifferent times. The node OUT switches between 400V and 0V according tothe pulse width modulation (PWM) signal 107. The gate driver 103 drivesM1 with gate control signal G1 and the gate driver 105 drives M2 withgate control signal G2. To turn on M2, the gate voltage G2 is driven toa voltage of OUT+10V to OUT+30V. To turn on M1, the gate voltage G1 isdriven to a voltage usually between 10V and 30V. The gate driver 105 isisolated from the input PWM signal with high voltage isolation barrier111.

The high voltage isolation technology could be of various types such asopto-isolators, transformers, capacitive isolation, or high voltagetransistors. Each of these isolation technologies have limitations whenextremely fast common mode transients are applied and can produceundesirable transients on the output. FIG. 2 illustrates a conventionaltiming diagram associated with the circuit of FIG. 1. The PWM signal 107is supplied to the input circuit 109, which supplies the control signalsto the gate driver 103 and through the high voltage isolation barrier111 to the gate driver 105. Note that OUT transitions slightly after G1goes low but before G2 goes high. Note also that for the falling edgeOUT transitions slightly after G2 goes low and before G1 goes high. OUTtransitions during the time G1 and G2 are both low.

FIG. 3 illustrates one example of distortions in the output signal thatcan be caused by common mode transients associated with high voltageisolation barriers. The transients 301 may cause the gate drive signalG2 to be distorted, which causes inaccuracy with respect to the inputPWM waveform. The transients 303 may cause M2 to be on at the same timeas M1, which can damage the power transistors and also lower efficiencyof the voltage conversion. The transients appear slightly after the OUTtransition and stop shortly after OUT transitions. Other combination ofevents can also result in undesirable transients in the output signal.

FIG. 4 illustrates an embodiment where opto-isolators may be used toprovide isolation between the control signal and the drivers and howcommon mode transients may arise in such circuits. When a current onnode 401 is applied to light emitting diode (LED) 403, the diode emitslight which is received by photo transistor 405. The output of the phototransistor may be used to drive a driver or supply some other circuit.The transient current I_(T), which charges or discharges capacitance411, may cause the input current to be too low to drive the LED 403 ormay cause the LED 403 to turn on when not desired if the transientcurrent is sufficiently strong. Thus, the common mode transient cancause the signal 407 on the receive side to differ from the input signalI_(IN) supplied on node 401. One solution to common mode transients inopto-isolators is to overdrive the LED 403 to keep it on when a commonmode transient occurs during an output high state and to take excesstransient current to ground during an output low state. That may requirean external switch, which could add cost and system complexity.

FIG. 5 illustrates timing considerations of common mode transientevents. Assume that the working voltage of the isolation technology isbetween 0V and 600V, then the transients are also between 0V and 600V.FIG. 5 illustrates four events 501, 502, 503, and 504. The length of theevent is determined by the slope, i.e., how long the transient takes togo between 0V and 600V. Note that while positive going transients areshown, negative going transients with a negative slope may also bepresent. The length Δt1 is associated with transient event 502 and Δt2is associated with transient event 504. The length of the transientevent can matter as the isolation channels may be natively immune totransient events greater than a certain length. For example, in theopto-system of FIG. 4, if the change in voltage is sufficiently slow(low slope), then the extra current I_(T) may not affect the diode. Thecurrent I_(T) is a function of the capacitance and the rate of change ofvoltage

$\left( {C\frac{v}{t}} \right)$

and if the rate of voltage change is sufficiently slow the current I_(T)may be insufficient to turn off the diode given a sufficiently highdrive current and will be insufficient to turn on the diode if I_(T) isless than the threshold current to turn on the diode. Other isolationtechnologies may also be immune to transient event lengths longer than aparticular time threshold.

Referring to FIG. 6, one embodiment utilizes a transient suppressioncircuit 601 to provide increased CMTI for a gate driver circuit for aswitching power supply. The transient suppression circuit 601 suppressestransients having an event time less than a predetermined length. Thus,if an isolation technology is natively immune to transients above aparticular threshold length, e.g., Δt2 for event 504 in FIG. 5, thetransient suppression circuit can be designed to suppress transientsbelow that particular threshold length. In that way, the suppressioncircuit suppresses transients of a duration corresponding to thethreshold length and below to ensure that those transients do not causethe output of the isolation system to be different from the input to theisolation system. Transients with an event length longer than thethreshold time may be ignored.

FIG. 7 illustrates an exemplary embodiment of the transient suppressioncircuit 601 that may be used in the system of FIG. 6. While a particularembodiment is illustrated in FIG. 7, other transient suppressioncircuits may be utilized in various embodiments described herein. Theisolation receiver 701 receives a signal transmitted across theisolation barrier. In a preferred embodiment the isolation receiver 701and the transient suppression circuit 601 are formed on the sameintegrated circuit die. The isolation barrier may be implemented as anopto-isolator, using transformers, using capacitive isolation, or withhigh voltage transistors. Referring to FIG. 8, the timing diagramillustrates operation of the transient suppression circuit. Thetransient suppression circuit 601 removes all transients that have anevent duration that is less than delay of T_(D), which correspond to thedelay of delay block 703. Thus, the suppression circuit can be tailoredfor the desired event length by adjusting the length of the delay block703. In some embodiments, the length of the delay block may beprogrammable. Thus, a delay control signal 709 from block 710, which maybe an input terminal, non-volatile memory, or some other location, maydetermine the length of the delay by, e.g., controlling a number ofdelay elements in the delay block. That allows the isolation technologyto be used in systems where different delays are desired. In addition aprogrammable system may be useful to allow determination, e.g., duringcharacterization or production testing, of the minimum delay required tosuppress common mode transients for a given system. In other embodimentsthe delay may be fixed.

Referring to FIGS. 7 and 8, the IN signal from the receive circuit 701has transients 803. The transients show up in the delayed IN_D signal at805 but not the RB signal, which ORs together the output IN_D of thedelay block and the IN signal from the receive circuit 701. Both theoriginal transients and the delayed transients show up in the SB signalat 807 and 809. Finally, the output of the gate driver OUT following thecross coupled NAND gates 705 is seen to be free of transients. Thus, thetransients causing the normally high signal to be low at 803 have beensuppressed. Positively going transients are shown at 811. The transients811 show up in the delay block output at 815 and in the output of RB at817 and 819. The transients do not show up at the output SB. Again, theoutput signal OUT to the gate driver following the cross coupled NANDgates 705 is seen to be free of transients. While a latching circuitusing cross coupled NAND gates is shown in the illustrated embodiment,any type of SR latch or even a flip-flop based circuit may be utilizedin other embodiments. The length of the delay block 703 has to besufficient to guarantee that desired transients events below aparticular length are suppressed.

The selected delay can be based in part on the desired CMTI. Forexample, assume a 0V to 600V working voltage. A CMTI of 30 kV/μs at 600Vcorresponds to a delay of 20 ns calculated based on

$\left( {{\left( \frac{30\mspace{14mu} {kV}}{\mu \; s} \right)\Delta \; t} = {600\mspace{14mu} V}} \right),$

where Δt corresponds to the delay of delay block 703. For a higher CMTI,the delay can be shorter. In one embodiment the delay is selected toensure a CMTI of at least 30 kV/μs. Other embodiments may select a CMTIof 200 kV/μs. Note that the calculations shown herein are simplified forillustration. For example, there may be a ringing response so a realsystem may choose a delay of 30 ns rather than the 20 ns shown above. Inaddition, in actual systems the choice of delay is typically based onthe maximum possible length of a CMTI failure that will be experienceddue to its inherent CMTI immunity. The delay should always be that longrather than just being based solely on a target CMTI rate. For example,for a system that targets 200 kV/μs CMTI, Δt would be 3 ns. However, theinherent CMTI immunity of the isolation channel may be, e.g., 50 kV/μs.That means that the minimum CMTI for which the native channel will failis 50 kV/μs, which corresponds to a pulse width of 12 ns, which islonger than 3 ns. So in order to determine the appropriate delay, nativechannel immunity also needs to be considered.

One aspect of the use of the transient suppression circuit is thatpropagation delay across the isolation technology is increased by thedelay associated with the delay circuit plus delay of the other logicgates in the transient suppression circuit. For opto-isolatorapplications with propagation delays of, e.g., 100-200 ns, an extra 20ns delay associated the suppression circuit would cause an approximate10 to 20 percent increase in the propagation delay. Some applicationsusing high voltage isolation can tolerate such additional propagationdelays and other applications, e.g., digital communication acrossisolation barriers, where propagation delays may be on the order of 10to 50 ns, may not be able to tolerate such application delays.

Referring to FIG. 7, for those applications that cannot tolerate theadditional propagation delay, the use of the transient suppressioncircuit may be bypassed in some embodiments. Thus, for thoseapplications that cannot tolerate the additional propagation delay, anembodiment may provide that the transient suppression circuit may bebypassed by selecting the bypass path through switch 711. The switchcontrol signal 715, sourced from block 716, which may be an inputterminal, non-volatile memory, or some other location, may be used tocontrol the bypass function. Other embodiments may omit the suppressionbypass logic entirely.

Referring to FIG. 9, illustrated is an embodiment of an improvedopto-isolator with a transient suppression circuit 601. The transientsuppression circuit 901 may be implemented as illustrated in FIG. 7.Rather than use an opto-isolator with an LED, an alternativeimplementation with similar IV characteristics to the opto-isolator canbe used in place of the opto-isolator illustrated in FIG. 9. In theembodiment illustrated in FIG. 10, the isolation link is implemented byproviding two dies 1001 and 1003 between which is a communication link1005. Each die 1001 and 1003 includes a transformer 1006 and 1008,respectively. Die 1001 includes transmit circuitry 1011 coupled totransformer 1006 and die 1003 includes receive circuitry 1015 coupled totransformer 1008. The signals are transmitted between the dies utilizingthe transformers 1006 and 1008 in each die and the magnetic couplingeffect between the transformers. In particular, transmit circuitry 1011drives the transformer 1006 with a signal such that energy is coupledfrom the primary to the secondary thereof. That allows energy to betransmitted on transmission lines 1005 that couple the transformers 1006and 1008 together. Each of the transformers is comprised of a primary1027 and a secondary 1029. Isolation barriers are present between theprimary and secondary windings of each transformer. The I/O terminals1017 and 1019 correspond to the terminals of an opto-isolator. The inputterminal 1017 may receive a current to indicate when to transmit acrossthe isolation barrier and terminal 1019 may be a ground connection.Other transformer embodiments may use a single transformer rather thanthe two transformers illustrated in FIG. 9. The transient suppressioncircuit 1030, which may be implemented as illustrated in FIG. 7,suppresses transient events having a length less than a preselectedevent length.

Rather than using transformer based isolation technology, FIG. 11illustrates use of the transient suppression circuit 1101 in acapacitive coupled isolation barrier. The transient suppression circuit1101 may be implemented as illustrated in FIG. 7. FIG. 12 illustrates anisolation technology in a Programmable Logic Controller (PLC) that hasmultiple inputs and multiple diodes, which may be used in industrialenvironments. The level of the voltage V_(IN) that causes the diode toturn on can be programmed with external resistors 1202 and 1204. Forease of illustrations, additional external resistors are not shown forthe other diodes. Rather than use diodes, an alternative isolationtechnology such as shown in FIG. 10 or 11 may be used. The receivecircuits 1201 on the receive side of the isolation barrier, whether thecommunication channel across the barrier is opto, inductive, orcapacitive, supply the received signals to transient suppressioncircuits 1205, which may be implemented in the manner illustrated inFIG. 7.

Thus, various approaches have been described relating to suppression oftransients associated with communication across an isolation barrier.The description of the invention set forth herein is illustrative, andis not intended to limit the scope of the invention as set forth in thefollowing claims. Other variations and modifications of the embodimentsdisclosed herein, may be made based on the description set forth herein,without departing from the scope of the invention as set forth in thefollowing claims.

What is claimed is:
 1. An apparatus comprising: a receive circuit toreceive signals from an isolation communication channel used tocommunicate across an isolation barrier and to provide a receive circuitoutput signal; and a common mode transient suppression circuit coupledto the receive circuit to suppress transients in the receive circuitoutput signal.
 2. The apparatus as recited in claim 1 wherein theisolation communication channel comprises a circuit to receive signalstransmitted from a light emitting diode.
 3. The apparatus as recited inclaim 1 wherein the isolation barrier utilizes capacitive isolation. 4.The apparatus as recited in claim 1 wherein the isolation channelincludes a transformer.
 5. The apparatus as recited in claim 1 where inthe common mode transient suppression circuit comprises a delay circuit.6. The apparatus as recited in claim 5 wherein the common mode transientsuppression circuit suppresses transients in the receive circuit outputsignal having an event length below a predetermined length of time. 7.The apparatus as recited in claim 6 wherein the predetermined length oftime corresponds to a delay associated with the delay circuit.
 8. Theapparatus as recited in claim 6 wherein the predetermined length of timeis less than approximately 50 nanoseconds.
 9. The apparatus as recitedin claim 6 wherein the predetermined length of time is less thanapproximately 30 nanoseconds.
 10. The apparatus as recited in claim 6wherein the apparatus has a common mode transient immunity of at least200 kV/microsecond.
 11. The apparatus as recited in claim 5 wherein thecommon mode transient suppression circuit further comprises: a firstlogic gate coupled to receive an input of the common mode transientsuppression circuit and is coupled to receive an output of the delaycircuit; a second logic gate coupled to receive the input of the commonmode transient suppression circuit and is coupled to receive the outputof the delay circuit; and a latching circuit coupled to the first andsecond logic gates.
 12. A method comprising; receiving a signaltransmitted to communicate across an isolation barrier; and suppressingtransients in the received signal having an event length less than apredetermined length of time.
 13. The method as recited in claim 12further comprising using a photodiode to communicate across theisolation barrier.
 14. The method as recited in claim 12 furthercomprises using capacitors to communicate across the isolation barrier.15. The method as recited in claim 12 further comprising usingtransformers to communicate across the isolation barrier.
 16. The methodas recited in claim 12 further comprising delaying the received signalin a delay circuit having a delay corresponding to the predeterminedlength of time.
 17. An isolation apparatus comprising: a transmitcircuit coupled on a transmit side of an isolation barrier to receive aninput signal and to supply a signal to be communicated to a receivecircuit on a receive side of the isolation barrier; the receive circuitto receive the signal on a receive side of the isolation barrier; and acommon mode transient suppression circuit coupled to the receive circuitto suppress transients in the signal received by the receive circuit ifthe transients have an event length less than a predetermined amount oftime.
 18. The apparatus as recited in claim 17 wherein a delayassociated with the delay circuit corresponds to the event length. 19.The apparatus as recited in claim 17 wherein a delay associated with thedelay circuit corresponds to the predetermined amount of time.
 20. Theapparatus as recited in claim 17 wherein the common mode transientsuppression circuit comprises a programmable delay circuit.